Amor asteroid

The Amor asteroids are a group of near-Earth asteroids named after the asteroid 1221 Amor. They approach the orbit of Earth from beyond, but do not cross it. Most Amors cross the orbit of Mars. The two moons of Mars, Deimos and Phobos, may be Amor asteroids that were captured by Mars's gravity.

The most famous member of this group is 433 Eros, which was the first asteroid to be orbited and then landed upon by a human probe (NEAR Shoemaker).

There are three general criteria which an asteroid must meet to be considered a member of the Amor asteroid class:

These three criteria boil down to a single test for membership: If an asteroid has a perihelion between 1.000 AU and 1.300 AU, it is an Amor asteroid. Any asteroid with this trait is considered an Amor-class asteroid, regardless of its semi-major axis, eccentricity, aphelion, inclination, physical properties, orbital stability, or place of origin.

An asteroid belongs to the Amor group if:

In summary, a > 1.0 AU and 1.017 AU < q < 1.3 AU.

While the above is the definition given in the source, the "a > 1.0 AU" constraint is not necessary because if the perihelion distance q is greater than 1.017 AU, then the semi-major axis a must also be greater than 1.017 AU (and greater than 1.0 AU).

There are 3729 Amor asteroids currently known. 580 of them are numbered, and 75 of them are named.

Amor asteroids can be partitioned into four subgroups, depending on their average distance from the Sun.

The Amor I subgroup consists of Amor asteroids whose semi-major axes are in between Earth and Mars. That is, they have a semi-major axis between 1.000 and 1.523 AU. Less than one fifth of Amor asteroids belong to this subgroup. Amor I asteroids have lower eccentricities than the other subgroups of Amors.

Some Amor I asteroids, such as 15817 Lucianotesi, do not cross the orbit of Mars. They can be considered a part of an Earth–Mars belt. However, not all asteroids located entirely between the orbits of Earth and Mars are Amors.

Amor I asteroids that do cross the orbit of Mars (like 433 Eros), do so from the inside.

Amor I asteroids that have semi-major axes very close to Earth's (such as 1992 JD) can be considered Arjuna asteroids because they have very low eccentricities and thus Earth-like orbits.

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